$ F = \left[\begin{array}{rr}1 & 2 \\ 5 & 1 \\ -1 & 5\end{array}\right]$ $ v = \left[\begin{array}{r}2 \\ -1\end{array}\right]$ What is $ F v$ ?
Answer: Because $ F$ has dimensions $(3\times2)$ and $ v$ has dimensions $(2\times1)$ , the answer matrix will have dimensions $(3\times1)$ $ F v = \left[\begin{array}{rr}{1} & {2} \\ {5} & {1} \\ \color{gray}{-1} & \color{gray}{5}\end{array}\right] \left[\begin{array}{r}{2} \\ {-1}\end{array}\right] = \left[\begin{array}{r}? \\ ? \\ ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ F$ , with the corresponding elements in column $j$ of the second matrix, $ v$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ F$ with the first element in ${\text{column }1}$ of $ v$ , then multiply the second element in ${\text{row }1}$ of $ F$ with the second element in ${\text{column }1}$ of $ v$ , and so on. Add the products together. $ \left[\begin{array}{r}{1}\cdot{2}+{2}\cdot{-1} \\ ? \\ ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ F$ with the corresponding elements in ${\text{column }1}$ of $ v$ and add the products together. $ \left[\begin{array}{r}{1}\cdot{2}+{2}\cdot{-1} \\ {5}\cdot{2}+{1}\cdot{-1} \\ ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{r}{1}\cdot{2}+{2}\cdot{-1} \\ {5}\cdot{2}+{1}\cdot{-1} \\ \color{gray}{-1}\cdot{2}+\color{gray}{5}\cdot{-1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{r}0 \\ 9 \\ -7\end{array}\right] $